Breast cancer is the most frequently diagnosed cancer and second leading cause of cancer death among women in the United States. While genetic, hormonal, biologic, and reproductive risk factors are important, as many as 75% of new cases occur in women with no known risk factors. Available data suggest that behavioral and psychosocial factors may play a role in breast cancer etiology and pathogenesis. A lack of well-designed population-based studies precludes firm conclusions regarding these associations, however. This proposal seek to utilize data from nearly 30 years of follow-up from the Alameda County Study, a population-based, prospective study of 9,202 adults (4,994 women) in Alameda County, CA, to identify behavioral and psychosocial factors that may be important in the development and progression of breast cancer. Proposed analyses specifically focus on the roles of physical activity, alcohol consumption, social isolation, social networks, depression, satisfaction, life events, and personal uncertainty in breast cancer risk. The Alameda County Study has collected a broad range of data on behavioral, social, psychological, and sociodemographic variables on a large sample of county residents in 1965, 1974, 1983, and 1994. Over 110,000 person-years of exposure currently are available for female participants. Linkage to the CA Tumor registry and the SEER Registry has enabled us to identify incident cases of breast cancer, with accompanying details on staging, histology, and treatments. This rich dataset, breadth of available information, and linkage to an established tumor registry present a unique opportunity to address important questions regarding associations between specific behavioral and psychosocial factors and increased risk of breast cancer. Results of these analyses will help clarify the potential etiologic impact of these important characteristics and may contribute to the development of new and more effective interventions for preventing the incidence and progression of breast cancer.